The present invention relates to a wafer chamfering method for working the peripheral edge portion of a semiconductor wafer, particularly, relates to a wafer chamfering method for working the peripheral edge portion of a semiconductor wafer into a convexly curved surface shape using a disk-shaped grindstone.
Conventionally, in a process for manufacturing a semiconductor device, in order to be able to facilitate the matching of the crystal orientation of a semiconductor wafer, in the peripheral edge of the wafer, there is formed an orientation flat which can be produced by cutting linearly part of the peripheral edge of the wafer, or a notch portion which can be produced by cutting part of the peripheral edge of the wafer in a substantially V-like or arc-like shape. Especially, the substantially V-shaped notch portion is most often employed because the V-shaped notch portion makes it possible to make efficient use of the limited area of the wafer and also can provide excellent positioning precision.
In the conventional semiconductor device manufacturing process, not infrequently, the peripheral edge of the semiconductor wafer is accidentally contacted with part of a device which is used in the semiconductor device manufacturing process. Such contact sometimes produces fragments or tips which are causes to deteriorate the quality of the semiconductor device. Therefore, it is necessary to chamfer the peripheral edge of the semiconductor wafer and, today, a wafer chamfering operation is generally employed in the semiconductor device manufacturing process.
Conventionally, to form a curved surface in the peripheral edge portion of a wafer, the rotational axis of a grindstone is moved in the thickness direction of the wafer along an orbit selected in consideration of the shape of the wafer peripheral edge portion after chamfered, such as a circular orbit around the peripheral edge portion of the wafer. And, in this operation, the grindstone is solely contacted with the peripheral edge of the wafer in such a manner as shown in FIG. 13, that is, the grindstone is supported such that it stands up straight or at right angles to the peripheral edge portion of the wafer at their mutual contact points, while the grinding surface of the grindstone is formed relatively sharp in order to be able to grind the notch portion of the wafer which is small in area. Therefore, it is inevitable that, in use, the sharp or tapered grinding surface of the grindstone can be worn or abraded and thus flattened in a short period of time.
Also, since, in the notch portion, the thickness of the grindstone is limited so as to be able to provide the sharp portion thereof as large as possible, from the viewpoint of strength, only the grindstone having a small diameter (that is, having a small circumference) can be used. Further, such small-diameter grindstone is not able to enhance the working efficiency thereof unless it is rotated at a very high speed, but such high-speed rotation of the grindstone causes the grinding surface of the grindstone to lose its predetermined allowable shape early. This is a vicious circle. If the grinding surface of the grindstone loses its predetermined allowable shape, then the shape of the working surface of the wafer is influenced by the thus shape-lost or deformed grinding surface. This raises the need to dress/adjust the deformed grinding surface again, which in turn increases the cost of the wafer as well as the cycle time of the wafer (that is, a unit time necessary to chamfer the peripheral edge portion of a piece of wafer). Similarly, from the viewpoint of preventing the grinding surface from losing its predetermined allowable shape, in the conventional wafer chamfering method, it is difficult that the same grindstone is commonly used to chamfer the circumferential portion of the wafer as well as to chamfer the notch portion thereof which is sharpened: that is, a series of chamfering operations to be executed continuously without replacing the grindstone cannot be applied in both of the circumferential portion and notch portion of the wafer.
Further, conventionally, a so called forming grindstone (a grindstone having a section which corresponds in shape to the shape of a workpiece or a wafer to be chamfered) is used in the mirror-surface working operation on the circumferential portion of a wafer, and the forming grindstone is rotated at a high speed of several thousands m/min. to thereby grind the circumferential portion of the wafer into a mirror-like surface. However, to provide such mirror-like surface, it is necessary to use a relatively soft grindstone having a micro abrasive grains. Therefore, the grindstone itself can be easily abraded partially to thereby lose its shape, and the shape loss of the grindstone is transferred to the wafer to thereby roughen the surface of the wafer. To avoid this, the grindstone must undergo a re-shaping/dressing operation frequently, which increases the cost of the wafer chamfering method.
Still further, in the conventional wafer chamfering method, because the relation between the peripheral speed of a wafer (that is, the moving speed of the peripheral edge of the wafer obtained as the wafer is rotated) and the peripheral speed of a grindstone (that is, the moving speed of the peripheral edge of the grindstone obtained as the grindstone is rotated) is not decided, grinding streaks are often produced in such a manner as to cause cracks in the wafer easily; and, in particular, when the peripheral speed of a wafer is low, the wafer is damaged extremely heavily when it is contacted with the grindstone.
Further, there is enforced a conventional method which uses float grains (so called as slurry) to polish the wafer. With use of this method, although the polished condition of the top surface portion of the wafer surface can be enhanced, the surface of the wafer cannot be finished in an arbitrary shape nor in the same shape all the time due to the fact that the polishing is depended upon the float grains being uncontrollable (see Japanese Patent Unexamined Publication No. Hei. 9-168953).